1. Field of the Invention
The present invention relates to a zoom lens. The present invention also relates to an image pickup apparatus having the same.
2. Description of the Related Art
In recent years, digital cameras designed to form an image of a target on the imaging surface of an image pickup element such as CCD or C-MOS sensor and electrically record the image have become popular to replace cameras of the type of exposing a film to light to form an image of a target. Digital cameras can be downsized if compared with cameras using a film by using a small image pickup element.
In the field of such digital cameras, so-called negative lead type zoom lenses that are advantageous for downsizing as disclosed in Patent Document 1 (JP-A-2004-318108), Patent Document 2 (JP-A-2004-318109) and Patent Document 3 (JP-A-2006-138969) are known. In a negative lead type zoom lens, a lens group of negative refractive power is arranged at the side closest to the object to be imaged, or the target.
Zoom lenses as disclosed in Patent Documents 1 through 3 have a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of positive refractive power and a fourth lens group of negative refractive power arranged in this order from the object side to the image side. An excellent symmetry of refractive power is realized with a structure for the entire zoom lens by forming the first lens group by means of two components of a negative lens component and a positive lens component while forming the second lens group by means of three lenses including a positive lens, a negative lens and a positive lens. This simple structure secures excellent optical performance.
However, recently, there is an increasing demand for image pickup apparatus having a zoom lens that is small but secures a good angle of view at the wide angle end and a sufficient variable power ratio.
In view of the above-identified problem, the present invention provide a zoom lens that is compact but advantageous for realizing a wide angle of view and/or a high variable power ratio and also an image pickup apparatus having such a zoom lens.
In view of the above-identified problem, a zoom lens according to the present invention has a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of positive refractive power and a fourth lens group of negative refractive power arranged in the above-mentioned order from the object side.
When the zoom lens is driven to zoom from the wide angle end to the telescopic end, at least the first lens group, the second lens group and the third lens group are driven to move and change the gaps separating the lens groups.
The first lens group is formed by two lens components including a negative lens component of negative refractive power and a positive lens component of positive refractive power that are arranged sequentially on the optical axis in the above-mentioned order from the object side to the image side with a gap separating them.
The second lens group includes a negative lens and a plurality of positive lenses. More preferably, the second lens group has three lenses including a positive lens, a negative lens and a positive lens that are arranged sequentially in the mentioned order from the object side to the image side. The lens components are lens bodies having only two refraction surfaces that contact air on the optical axis as object side surface and image side surface.
When a zoom lens according to the present invention has a focusing feature, it is focused at infinity at the time of shipment. This statement applies to all the compositional requirements and all the conditions shown hereinafter.
With this basic arrangement, the number of lens groups and the total number of lenses can be reduced and the image pickup apparatus can be downsized advantageously by arranging a lens group of negative refractive power at the object side.
Additionally, the principal point is placed at an object side position to make it advantageously possible to reduce the diameter of the first lens group and the total length of the zoom lens by forming the first lens group by two lens components as described above.
The second lens group is made to mainly take the role of a variator, while the first lens group and the third lens group take the role of a compensator and that of reducing the fluctuations of the exit pupil at the time of varying power as they are driven to move. Additionally, this arrangement is advantageous for correcting the field curvature.
As the fourth lens group of negative refractive power is placed at the image side relative to the third lens group, an excellent symmetry of refractive power is realized in the zoom lens so that aberrations can be corrected with ease in the entire zooming range. Particularly, this arrangement is advantageous for reducing the off-axial aberrations such as the field curvature at the wide angle end and the astigmatism.
The refractive power needs to be increased as the variable power ratio is raised in order for the second lens group to take the role of a variator. However, when the refractive power is simply increased, the aberrations are apt to fluctuate at the wide angle end and at the telescopic end. With the basic arrangement of the present invention, the second lens group is formed by means of three lenses including a positive lens, a negative lens and a positive lens arranged sequentially in the above-mentioned order from the object side, paying attention to the symmetry of refractive power of the zoom lens. Thus, as a result, an excellent symmetry of refractive power is realized in the second lens group and fluctuations of the aberrations at the wide angle end and at the telescopic end and the appearance of aberrations on the axis at the telescopic end can advantageously be suppressed.
A zoom lens in the first aspect of the present invention further includes a fourth lens group formed by a single negative lens component whose concave is made to face the object side in addition to the above-described basic arrangement and the negative lens component in the first lens group and the negative lens component in the fourth lens group are made to satisfy the requirements of conditional formula (1) shown below:−0.80<rG1nr/rG4nf<−0.40  (1),where                rG1nr is the radius of paraxial curvature of the image side surface of the negative lens component in the first lens group; and        rG4nf is the radius of paraxial curvature of the object side surface of the negative lens component in the fourth lens group.        
The conditional formula (1) specifies a favorable profile of the negative lens component of the first lens group and the fourth lens group of negative refractive power. When the image side concave of the negative lens component arranged at the extremity of the object side and the object side concave of the fourth lens group are made to satisfy the above requirements, the exit pupil can be arranged remotely from the image surface, while maintaining the symmetry. Then, as a result, the quantity of peripheral light can be prevented from being small and the color shading can be suppressed advantageously, while excellently correcting both the axial aberrations and the off-axial aberrations.
An excellent symmetry can be realized and particularly the off-axial aberrations can be reduced to make it possible to advantageously broaden the angle of view when the above upper limit is not exceeded.
Advantageously, the off-axial aberrations of higher order can be suppressed and the quantity of peripheral light can be secured, while the color shading can be reduced when the above lower limit is not exceeded and the curvature absolute value on the object side concave of the fourth lens group is suppressed. Additionally, the negative refractive power of the first lens group can be advantageously secured and a broadened angle of view and an increased variable power ratio can advantageously be realized.
In a zoom lens in the second aspect of the present invention, the negative lens component of the first lens group includes negative lenses that satisfy the requirements of conditional formula (2) shown below with the above-described basic arrangement:1.78<ndG1n<2.40  (2),    where            ndG1n is the refractive index of any of the negative lenses in the negative lens component of the first lens group at d line.        
The conditional formula (2) specifies a favorable refractive index of any of the negative lenses in the negative lens component of the first lens group. When the requirements of the above conditional formula are satisfied, the absolute value of the radius of curvature of each of the lens surfaces of the negative lens component of the first lens group can be reduced to advantageously make it possible to reduce the aberrations and secure the negative refractive power in the first lens group with ease at the same time. Additionally, the symmetry of profile of the negative lens component of the first lens group and the negative lens component of the fourth lens group can easily be maintained to advantageously make it possible to reduce the off-axial aberrations.
The negative refractive power of the first lens group can be secured and various aberrations can be reduced at the same time to advantageously make it possible to realize a broadened angle of view and an increased variable power ratio when the above lower limit is not exceeded.
The cost of the lens materials can advantageously be reduced when the above upper limit is not exceeded.
The negative lens component of the first lens group may include negative lenses that satisfy the requirements of the conditional formula (2) in the zoom lens in the first aspect of the present invention. Then, it is even more advantageously possible to realize a broadened angle of view and an increased variable power ratio.
The fourth lens group may be formed only by a single negative lens component in a zoom lens in the second aspect of the present invention. The function of the fourth lens group is mainly directed to correction of the field curvature. For this reason, the number of lens components preferably is reduced to make it advantageously possible to downsize the zoom lens.
In either aspect of the present invention, one or more than one the compositional requirements and the requirements of the conditional formulas are preferably satisfied at the same time.
Preferably, both the image side surface of the negative lens component of the first lens group and the object side surface of the fourth lens group are concave aspheric surfaces. When they are concave aspheric surfaces, the off-axial aberrations of higher orders can further advantageously be reduced at the wide angle end to advantageously ensure an excellent optical performance when a broadened angle of view and an increased variable power ratio are realized. Furthermore, when both the object side surface of the negative lens component of the first lens group and the image side surface of the fourth lens group are aspheric surfaces, the off-axial aberrations of higher order and the axial aberrations can advantageously be corrected by the four aspheric surfaces.
Both the negative lens component and the positive lens component of the first lens group are respectively formed by single lenses to advantageously reduce the weight and the cost of the first lens group.
Both the third lens group and the fourth lens group preferably respectively have single lens components. Then, the symmetry of refractive power for the set of the negative lens component and the positive lens component of the first lens group and the set of the third lens group and the fourth lens group can be established by a few number of the lens components to advantageously make it possible to downsize the zoom lens and ensure an excellent optical performance at the same time.
Additionally, both the third lens group and the fourth lens group preferably are formed by respective single lenses. Then, the size of the zoom lens can advantageously be reduced when the lens barrel is retracted and the zoom lens can be manufactured at low cost.
A zoom lens according to the present invention preferably satisfies the requirements of conditional formula (3) shown below:0.1<DG1np/DG3G4<1.0  (3),    where            DG1np is the distance between the negative lens component and the positive lens component of the first lens group on the optical axis; and        DG3G4 is the distance between the third lens group and the fourth lens group on the optical axis at the telescopic end.        
Thus, the first lens group of a zoom lens according to the present invention is formed by arranging a negative lens component and a positive lens component sequentially from the object side in the above-mentioned order to make it advantageously possible to reduce the size of the first lens group in diametrical directions and also the size of the entire zoom lens by placing the principal point of the first lens group at an object side position. Then, the field curvature can advantageously be corrected as the two lens components are arranged with a space separating them.
The conditional formula (3) specifies a preferable ratio of the air gap in the first lens group and the gap between the third lens group and the fourth lens group at the telescopic end. An excellent symmetry of the entire zoom lens is realized at the telescopic end to make it advantageously possible to correct the axial aberrations and the off-axial aberrations at the telescopic end.
An excellent symmetry is achieved for the optical system when the above lower limit is not exceeded. Additionally, a space for allowing the third lens group to move is secured when the third lens group needs to be driven to move for focusing.
The symmetry of the zoom lens can advantageously be maintained and made flat when the lens barrel is retracted by suppressing the air gap in the first lens group to an appropriate level provided that the above upper limit is not exceeded.
Preferably, the negative lens component of the first lens group satisfies the requirements of conditional formula (4) shown below:−1.0<(rG1nf+rG1nr)/(rG1nf−rG1nr)<0.75  (4),where                rG1nf is the radius of paraxial curvature of the object side surface of the negative lens component of the first lens group; and        rG1nr is the radius of paraxial curvature of the image side surface of the negative lens component of the first lens group.        
The conditional formula (4) specifies a preferable profile of the negative lens component of the first lens group.
Advantageously, the curvature of the object side surface can be reduced when the above lower limit is not exceeded and both the field curvature at the wide angle end and the spherical aberration at the telescopic end can be reduced by reducing the curvature of the image side surface when the above upper limit is not exceeded.
Preferably, the second lens group satisfies the requirements of conditional formula (5) shown below:0.10<fG2/ft<0.55  (5),where                fG2 is the focal length of second lens group; and        ft is the focal length of the entire optical system of the zoom lens at the telescopic end.        
The conditional formula (5) specifies a preferable refractive power level of the second lens group.
The axial aberrations can advantageously be reduced at the telescopic end by suppressing the refractive power of the second lens group to an appropriate level when the above lower limit is not exceeded.
The second lens group can be made to have a satisfactory variable power function by securing refractive power for the second lens group when the above upper limit is not exceeded to make it advantageously possible to reduce fluctuations of the aberrations both at the telescopic end and at the wide angle end and also reduce the size of the zoom lens when the lens barrel is retracted.
Preferably, the first lens group satisfies the requirements of conditional formula (6) shown below:−0.70<fG1/ft<−0.20  (6),where                fG1 is the focal length of first lens group; and        ft is the focal length of the entire optical system of the zoom lens at the telescopic end.        
It is preferably to secure a sufficient level for the negative refractive power of the first lens group in order to realize a wide angle of view and/or a high variable power ratio. Since an excellent symmetry of refractive power is realized in a zoom lens according to the present invention, the zoom lens can be formed that the aberrations can be corrected with ease when a sufficient level is secured to the refractive power of the first lens group. The conditional formula (6) specifies a preferable level of refractive power for the first lens group.
Thus, the angle of view can be broadened and the variable power ratio can advantageously be raised in a zoom lens according to the present invention by securing the refractive power of the first lens group when the above lower limit is not exceeded.
Additionally, the off-axial aberrations can be reduced at the wide angle end by suppressing excessive negative refractive power when the above upper limit is not exceeded.
Preferably, the second lens group satisfies the requirements of conditional formula (7) shown below:0.01<σG2/ft<0.25  (7),where                σG2 is the thickness of the second lens group on the optical axis; and        ft is the focal length of the entire optical system of the zoom lens at the telescopic end.        
The conditional formula (7) specifies a preferable thickness of the second lens group on the optical axis.
The thickness of the zoom lens can advantageously be reduced when the lens barrel is retracted provided that the above upper limit is not exceeded.
The lens can be prepared with ease and both the field curvature and the spherical aberration can be corrected with ease by securing a satisfactory thickness when the above lower limit is not exceeded.
Preferably, a zoom lens according to the present invention satisfies the requirements of conditional formula (8) shown below:3.0<ft/fw<20.0  (8),where                ft is the focal length of the entire optical system of the zoom lens at the telescopic end; and        fw is the focal length of the entire optical system of the zoom lens at the wide angle end.        
The conditional formula (8) specifies a preferable zooming ratio of the zoom lens.
The zoom lens can accommodate various scenes to be shot when the above lower limit is not exceeded. However, if the zooming ratio exceeds the above lower limit, a three-group zoom lens or a two-group zoom lens can ensure a satisfactory optical performance.
The number of lens groups in the zoom lens and/or the entire length of the zoom lens can be suppressed when the above upper limit is not exceeded.
A zoom lens according to the present invention preferably is a four-group zoom lens having four lens groups including a first lens group, a second lens group, a third lens group and a fourth lens group. Four lens groups can realize an excellent symmetry of refractive power to advantageously make it possible to downsize the zoom lens and ensure an excellent optical performance at the same time.
Preferably, a zoom lens according to the present invention has an aperture stop that moves integrally with the second lens group when zooming from the wide angle end to the telescopic end. Such an arrangement is advantageous for reducing the diameter of the second lens group. The diameter of the flux of light in the second lens group can be reduced to make it advantageously possible to downsize the zoom lens and reduce the aberrations. While the aperture stop may be arranged at the object side or at the image side of the second lens group or even in the second lens group, advantageously the off-axial aberrations can be corrected and the thickness of the second lens group can be reduced at the same time when the aperture stop is arranged at the image side of the second lens group.
Preferably, an operation of shifting the focus from a remote object to a nearby object is conducted by moving only the third lens group toward the object side. Since the third lens group can be downsized and its focus driving range can be secured with ease, it is preferable to move this lens group when shifting the focus.
Preferably, the third lens group includes a cemented lens component having a negative lens and a positive lens. Then, the chromatic aberration of the third lens group can advantageously be corrected. Particularly, when the third lens group is a focusing lens group, the chromatic aberration of magnification can advantageously be reduced at the time of shifting the focus to a nearby object at the telescopic end.
Preferably, the fourth lens group is formed by a plastic single lens. Advantageously, both the cost and the weight of the lens barrel can be reduced by forming the fourth lens group by a plastic single lens. Additionally, the fourth lens group can be processed with ease when it is provided with an aspheric surface.
An image pickup apparatus according to the present invention has a zoom lens and an imaging plane arranged at the image side of the zoom lens as well as an image pickup element for converting the optical image on the imaging plane into electric signals, and the zoom lens thereof is a zoom lens as defined above.
Advantageously, a zoom lens as defined above can be downsized and the angle of view can be broadened while the variable power ratio can be raised and the exit pupil can be moved away from the image plane with ease. Therefore, such a zoom lens can be adopted for the imaging zoom lens of an image pickup apparatus.
Additionally and preferably, the fourth lens group is fixed relative to the imaging plane when zooming from the wide angle end to the telescopic end. When the fourth lens group is fixed, the mechanical configuration of the image pickup apparatus can be dramatically simplified to make it possible to reduce the thickness of the image pickup apparatus when the lens barrel is retracted and also reduce the cost.
An image pickup apparatus according to the present invention preferably has an image conversion section for converting the electric signals from the zoom lens that includes distortion into video signals in which the distortion is corrected by way of an image processing operation. An image picked up by a zoom lens is apt to produce a tradeoff relationship between distortion and astigmatism at the wide angle end. Therefore, the zoom lens can be downsized and the quality of the recorded image can be improved by optically correcting the astigmatism and electrically correcting the barrel-shaped distortion that is apt to be produced at the wide angle end before recording and/or displaying the image.
An image pickup apparatus according to the present invention preferably has an image conversion section for converting the electric signals from the zoom lens that includes chromatic aberration of magnification into video signals that are corrected by way of an image processing operation. For example, when distortion is electrically corrected on a color signal by color signal basis, the chromatic aberration of magnification is consequently corrected. Then, as a result, the chromatic aberration of magnification of the zoom lens itself can be shared to make it advantageously possible to reduce the cost of the materials and downsize the image pickup apparatus.
A zoom lens as defined below may be provided. Namely, there is provided a zoom lens including a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of positive refractive power and a fourth lens group of negative refractive power arranged sequentially from the object side toward the image side in the above-mentioned order,                at least the first lens group, the second lens group and the third lens group being driven to move, the gaps separating the lens groups being made to change when zooming from the wide angle end to the telescopic end;        the first lens group being formed by two lens components including a negative lens component of negative refractive power and a positive lens component of positive refractive power arranged on the optical axis from the object side toward the image side in the above mentioned order with a gap separating them;        the second lens group including a negative lens and a plurality of positive lenses but preferably including three lenses of a positive lens, a negative lens and a positive lens arranged sequentially from the object side toward the image side in the above mentioned order;        the fourth lens group being formed by a single negative lens component; and        the zoom lens satisfying the requirements shown below:4.0<ft/fw<20.0  (8-1),where        ft is the focal length of the entire optical system of the zoom lens at the telescopic end, and        fw is the focal length of the entire optical system of the zoom lens at the wide angle end.        
A zoom lens as defined below may be provided. Namely, there is provided a zoom lens including a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of positive refractive power and a fourth lens group of negative refractive power arranged sequentially from the object side toward the image side in the above mentioned order,                at least the first lens group, the second lens group and the third lens group being driven to move while the fourth lens group is fixed, the gaps separating the lens groups being made to change when zooming from the wide angle end to the telescopic end;        the first lens group having a negative lens component arranged at the object side extremity;        the second lens group including a negative lens and a plurality of positive lenses but preferably including three lenses of a positive lens, a negative lens and a positive lens arranged sequentially from the object side toward the image side in the above mentioned order;        the fourth lens group including a negative lens component with its concave surface directed to the object side and the total number of lens components in the fourth lens group being one; and        the negative lens component in the first lens group and the negative lens component in the fourth lens group satisfying the requirements of conditional formula (1) shown below:−0.80<rG1nr/rG4nf<−0.40  (1),where        rG1nr is the radius of paraxial curvature of the image side surface of the negative lens component in the first lens group; and        rG4nf is the radius of paraxial curvature of the object side surface of the negative lens component in the fourth lens group.        
A zoom lens as defined below may be provided. Namely, there is provided a zoom lens including a first lens group of negative refractive power, a second lens group of positive refractive power, a third lens group of positive refractive power and a fourth lens group of negative refractive power arranged sequentially from the object side toward the image side in the above-mentioned order,                at least the first lens group, the second lens group and the third lens group being driven to move, the gaps separating the lens groups being made to change when zooming from the wide angle end to the telescopic end;        the first lens group having a negative lens component arranged at the object side extremity;        the second lens group including a negative lens and a plurality of positive lenses but preferably including three lenses of a positive lens, a negative lens and a positive lens arranged sequentially from the object side toward the image side in the above mentioned order;        the fourth lens group including a negative lens component with its concave surface directed to the object side, the total number of lens components in the fourth lens group being one; and        the zoom lens satisfying the requirements of conditional formula (8-1) shown below:4.0<ft/fw<20.0  (8-1),where        ft is the focal length of the entire optical system of the zoom lens at the telescopic end, and        fw is the focal length of the entire optical system of the zoom lens at the wide angle end.        
Preferably, each of the above-defined arrangements satisfies the plurality of requirements at the same time.
Each of the conditional formulas can be modified as follows to make its effect more reliable.
As for the conditional formula (1),                preferably, the lower limit is −0.75, more preferably −0.72; and        preferably, the upper limit is −0.50, more preferably −0.60.        
As for the conditional formula (2),                preferably, the lower limit is 1.80, more preferably 1.85; and        preferably, the upper limit is 2.10, more preferably 1.90.        
As for the conditional formula (3),                preferably, the lower limit is 0.2, more preferably 0.3; and        preferably, the upper limit is 0.8, more preferably 0.6.        
As for the conditional formula (4),                preferably, the lower limit is 0.1, more preferably 0.5; and        preferably, the upper limit is 0.73, more preferably 0.70.        
As for the conditional formula (5),                preferably, the lower limit is 0.30, more preferably 0.40; and        preferably, the upper limit is 0.45, more preferably 0.50.        
As for the conditional formula (6),                preferably, the lower limit is −0.60, more preferably −0.55; and        preferably, the upper limit is −0.30, more preferably −0.40.        
As for the conditional formula (7),                preferably, the lower limit is 0.10, more preferably 0.20; and        preferably, the upper limit is 0.24, more preferably 0.23.        
As for the conditional formulas (8) and (8-1),                preferably, the lower limit is 4.0, 4.5, more preferably 4.8; and        preferably, the upper limit is 10.0, 8.0, more preferably 6.0.        
As may be clear from the above description, the present invention can provide a zoom lens that is small but advantageous for realizing a wide angle of view and/or a high variable power ratio and also an image pickup apparatus having such a zoom lens.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly includes the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.